1. Field of the Invention
This invention relates generally to spin valve transistor structures for magnetic read heads and more particularly to an integral spin valve transistor (SVT) structure formed in a semiconductor slider without the usual SVT wafer bonding step.
2. Description of the Related Art
Magneto-electronic devices with improved magnetic sensitivity are vital to increasing magnetic data storage densities. Giant magnetoresistance (GMR) sensors using only two layers of ferromagnetic (FM) material, the free (sensing) layer and the pinned (reference) layer, separated by a layer of non-magnetic conductive material (e.g., copper) are generally referred to as spin valve (SV) sensors and represent a major improvement that relies on the spin-dependent scattering effect. In SV sensors, the resistance of a spin valve (SV) to a sense current varies as a function of the spin-dependent transmission of the conduction electrons between two magnetic layers separated by a nonmagnetic spacer layer and the accompanying spin-dependent scattering that occurs at the interface of the magnetic and non-magnetic layers and within the magnetic layers. The SV stack may operate with sense current-in-the-plane (CIP) of the layers or with sense current-perpendicular-to-plane (CPP) of the layers.
Later, a new class of MR sensors, herein denominated tunnel-valve (TV) sensors, was discovered in which the nonmagnetic layer separating the two FM layers is made with an ultrathin nonconductive material, such as an aluminum oxide layer <20 Å thick. A distinctive feature of the TV sensor is its high impedance (>10 Σ−:m2), which allows for large signal outputs. A TV stack has two FM layers separated by a thin insulating tunnel barrier layer whose operation relies on the spin-polarized electron tunneling phenomenon known in the art. For low applied fields (<100 Oe), the pinned (reference) layer is essentially fixed in one direction because of, for example, a higher coercivity, than the free (sensing) layer, which is essentially free to rotate in response to external fields. The insulating tunnel barrier layer is thin enough so that quantum mechanical tunneling occurs between the two FM layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the TV a function of the relative moment orientations and spin polarizations of the two FM layers. The TV stack operates with sense current-perpendicular-to-plane (CPP) of the layers.
Meanwhile, the functional integration of the semiconductor and the ferromagnet resulted in a new magneto-electronic device denoted the spin valve transistor (SVT). The SVT is a three-terminal device, analogous to a metal-base transistor, where the charge carrier populations are distinguished by spin (magnetic moment) instead of electrical charge. The metal SVT base, which includes a spin-selective element such as the CPP SV stack, is sandwiched between two N-type silicon layers, for example. Hot electrons are injected over a Schottky barrier from the SVT emitter into the SV stack. Those spin-oriented electrons that are not spin-scattered continue through the SV stack and traverse a second Schottky barrier at the SVT collector, forming the magnetocurrent, which is extremely sensitive to the spin-scattering in the SVT. The emitter and collector junction materials are usually selected to provide a higher Schottky barrier at the emitter junction (e.g., silicon-platinum) than at the collector junction (e.g., silicon-gold). Until recently, successful fabrication of such metal-silicon SVT junctions required growing the metal layers directly on the crystalline semiconductor surface because any oxide contamination at the junction suppresses the tunneling of spin-oriented electrons into the conduction band of the semiconductor, and inhibits interdiffusion. This means that two semiconductor wafers must be bonded face-to-face at some point during fabrication to complete the SVT stack. Spin-scattering of hot electrons is determined by the magnetic state of the SV stack, which is sensitive to the intensity and orientation of an external magnetic field. Thus, a SVT may be employed as a read sensor for magnetic data storage systems because of the resulting magnetocurrent sensitivity to external magnetic fields, which provides a relative magnetic response of over 300% at room temperature with small fields.
The first SVT was reported by Mark Johnson (“Bipolar Spin Switch,” Science, Vol. 260, pp. 320–323, Apr. 16, 1993), who describes a basic device with no power gain and nanovolt signal levels. Johnson later describes a spin-injected field-effect transistor (FET) in U.S. Pat. No. 5,654,566. Subsequently, Monsma et al. (“Perpendicular Hot Electron Spin-Valve Effect in a New Magnetic Field Sensor: The Spin-Valve Transistor,” Phys. Rev. Lett. Vol. 74, No. 26, pp. 5260–3, Jun. 26, 1995) describe the current-perpendicular-to-plane (CPP) SVT design now well-known in the art. Monsma et al. emphasize the importance of direct silicon-metal interfaces at the emitter and collector junctions, and propose a detailed method for direct semiconductor bonding through spontaneous adhesion of a gold bonding layer to join two silicon-metal Schottky junctions into a single SVT stack.
Later, Kumar et al. (“The Spin-Valve Transistor,” J. Phys. D:Appl. Phys., Vol. 33, pp. 2911–20, November 2000) assert that it is essential to grow the metal layers directly on a crystalline silicon layer. They propose a vacuum metal-bonding method requiring clean room robots and a noble-metal bonding layer to join two processed wafers face-to-face during their SVT fabrication procedure. FIG. 1 shows the typical spin-valve transistor (SVT) embodiment 20 described by Kumar et al. A forward-biased emitter 22 having an emitter current IE and a reverse-biased collector 24 having a collector current IC each form a Schottky barrier with the base layer 26, which has a base current IB. The Si—Pt Schottky barrier 28 and the Si—Au Schottky barrier 30 are formed at emitter 22 and collector 24, respectively. Base layer 26, shown in detail, includes a NiFe/Au/Co SV stack 32. SVT 20 is fabricated by first building up all of the elements in base layer 26 on crystalline collector layer 24, using conventional thin-film techniques. Then a second crystalline silicon emitter layer 22 is bonded to base layer 26 at junction 28 using a sophisticated vacuum metal-bonding method. Once this surface bonding effort forms the platinum-to-platinum bonding layer 34, additional conventional thin-film fabrication steps may be employed to etch and mill emitter layer 22 to form a plurality of SVT structures exemplified by SVT 20. In FIG. 1, the magnetocurrent IM is shown passing through from emitter to collector and a current monitor in the emitter-base circuit measures a related sense current IS.
Other practitioners have proposed SVT fabrication methods that do not require a spontaneous adhesion bonding step. For example, in U.S. Pat. No. 5,973,334, Mizushima et al. employ a buffered nonmagnetic metal layer (e.g., aluminum) at the emitter to inject hot electrons into the SV stack (base) instead of using a Schottky barrier. This means that a single Schottky junction at the SVT collector is sufficient for SVT operation and no bonding step is needed to bond to an emitter Schottky junction for their SVT. In U.S. Pat. No. 6,218,718 B1, Gregg et al. describe a SVT wherein the pinned layer structure of the SV stack is separated by a silicon PN junction from a cobalt-oxide layer. Although Gregg et al. teach using a silver layer for the direct adhesion bonding step necessary for fabrication of one embodiment of their SVT design, they also suggest depositing a polysilicon layer at the collector to create an ohmic junction instead of a second Schottky junction. In this case, a single Schottky junction at the emitter is sufficient for SVT operation, and no bonding step is needed to bond to a collector Schottky junction for their SVT.
Clearly, the SVT is very useful as a read sensor for magnetic data storage systems because of its unusual sensitivity to external magnetic fields at room temperature and its small read width (RW) dimension. The SVT may be used as a read sensor in a magnetic read/write head for transferring data between a control circuit and a moving magnetic storage medium, such as a rotating magnetic disk or a streaming magnetic tape. Such a magnetic head is usually mounted on a slider at an air bearing surface (ABS) disposed at the moving surface of the magnetic medium. As is well-known, the slider is a key factor in determining the efficiency, density, speed, and accuracy with which data can be transferred and stored in a magnetic recording media in a data storage drive. The slider's size is an important performance factor; smaller sliders can effectively increase the recording medium's storage capacity by storing data more compactly on the recording medium. Functional integration is also an important performance factor. The typical slider merely includes a magnetic head to write and read binary data at a magnetic recording medium. However, some practitioners describe advanced slider designs that incorporate other circuitry to perform additional functions close to the read/write elements.
For example, in U.S. Pat. No. 4,809,103, Lazzari describes a silicon wafer having a magnetic head and an electronic circuit integrated to the head. As another example, Beck et al. (J. W. Beck & B. Brezoczky, “Magnetic Head Assembly Including Head Circuitry,” IBM Technical Disclosure Bulletin, Vol. 22, No. 1, June 1979.) refer to a magnetic head assembly with input and output signal-processing circuitry incorporated in a chip fixed to an air bearing slider. The circuitry of Beck et al. appears to include read signal preamplifier and write signal output electronics. In addition to the improvement of signal-to-noise ratios for sensor signals, another benefit of disposing support circuitry aboard a slider is compactness. In modern data storage drives, element size is critical, especially in particularly compact applications such as data storage drives for laptop computers.
There is a clearly-felt need in the art for a monolithic slider that integrates a read/write head with an integral SVT and other signal-processing functions in a single element that may be fabricated using only those thin-film fabrication methods that are well-understood and widely-appreciated in the art. Until now, there was no method known in the art for fabricating a slider incorporating an integral SVT and other electronic components in a conventional thin-film process. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.